Method for monitoring a system comprising a number of readers and a plurality of portable communication units

ABSTRACT

A method for monitoring a system includes reading devices and a plurality of portable communication units, wherein during operation of the system, for a communication unit there are carried out a plurality of data transactions between at least one reading device and the communication unit via a corresponding communication interface. Within the framework of a data transaction a first data set is transferred from a reading device to the communication unit and stored. A second data set stored in the communication unit is transferred to the reading device. Chaining occurs with first and second data sets of the respective data transactions are formed so that in two consecutive data pairs the first data set of the one data pair corresponds to the second data set of the other data pair, and a criterion is checked for being fulfilled.

The invention relates to a method for monitoring a system comprising anumber of reading devices and a plurality of portable communicationunits.

From the prior art there are known systems with which utilized servicesare captured via a data exchange between a reading device and a portablecommunication unit carried by the user, so that these services can bebilled to the user at a later time. Systems for capturing services areknown in particular from the field of public transport. Viacorresponding reading devices in means of transport, which communicatewith the communication units of users, the travelled paths are capturedhere.

In the just described systems there are often employed RFID tags asportable communication units, which contactlessly communicate with acorresponding reading device. In many applications there are usedUHF-RFID tags which can be queried by a corresponding RFID readingdevice and send back their identity by modulation of the high-frequencyenergy scattered back at the antenna of the tag (backscatter). As UHFtags have no power supply of their own, they are normally of a simpleconstruction and only insufficiently protected against forgery.

In the print A. Arbit, J. Oron, A. Wool, Toward Practical Public KeyAnti-Counterfeiting for Low-Cost EPC Tags, 2011, IEEE InternationalConference on RFID, there is described a protocol based on a public keycryptography for the secured identification of RFID tags. This protocolprevents the creation of new tags, but does not protect against themanufacturing of duplicates of already existing tags.

In the print U.S. Pat. No. 6,766,161 B2 there is described a method forrecognizing duplicated communication devices based on transaction codes.Here, in a host and a communication device there is held a sequence ofconsecutive transaction codes, upon each communication between host andcommunication device there being transmitted a new transaction code ofthe sequence to the host. The host then checks whether the transmittedtransaction code matches the transaction code which is to be expectedaccording to the sequence deposited at the host.

It is the object of the invention to improve the operation of a systemcomprising a number of reading devices and a plurality of portablecommunication units to the effect that an efficient protection againstthe duplication of communication units is guaranteed.

This object is achieved by the method according to claim 1 and thesystem according to claim 13. Developments of the invention are definedin the dependent claims.

The method according to the invention serves for monitoring a systemcomprising a number of reading devices and a plurality of portablecommunication units. During operation of the system, for a givencommunication unit, which can represent an arbitrary communication unitof the plurality of communication units, there are carried out aplurality of data transactions between at least one reading device andthe given communication unit via a corresponding communicationinterface. If the system includes several reading devices, alsodifferent reading devices can be involved in the data transactions. Anindividual data transaction, however, is always carried out between areading device and the given communication unit. Preferably, thecommunication interface between reading device and communication unit isa contactless interface and the portable communication units representaccordingly configured transponders which communicate with suitablecontactless reading devices. In particular, the transponders are theabove-mentioned RFID transponders or UHF-RFID transponders.

Within the framework of a data transaction, during operation of thesystem there is transmitted a first data set from a reading device tothe given communication unit and stored there. Further, a second dataset, which was stored in the given communication unit upon the mostrecently carried out data transaction, is transferred to the readingdevice. A data transaction carried out is understood to be here a datatransaction which was completed, i.e. for which the storing of a firstdata set as well as the transfer of a second data set was carried out.

In the method according to the invention there is carried out a suitableevaluation of the data sets transferred in the system, with a computerunit which can be configured e.g. as a central server. In so doing, achaining of data pairs comprising the first and second data set of therespective data transactions is formed in such a way that for twoconsecutive data pairs of the chaining the first data set of the onedata pair corresponds to the second data set of the other data pair.This chaining can also be interrupted, where applicable. Upon formingthe chaining there is checked a criterion which is fulfilled when thechaining has several parallel chains of data pairs or when two datapairs are found which include different first data sets while the seconddata set is the same.

Parallel chains are understood to be here chains of data pairs which areindependent of each other and/or at least partly temporally overlapping.The chains contain respectively consecutive data pairs according to theabove definition, i.e. for two consecutive data pairs the first data setof the one data pair corresponds to the second data set of the otherdata pair. Nevertheless, a chain can also be interrupted at one orseveral points. The occurrence of parallel chains allow the conclusionthat two identical communication units exist side by side and carry outdata transactions which are not connected to each other. Upon therecognition of such chains or of data pairs having different first andthe same second data sets, a duplication of the given communication unitis recognized by the computer unit. As a result, corresponding countermeasures can be initiated. In a preferred embodiment, the communicationunit in the system will be blocked from being further employed.

The method according to the invention has the advantage that duplicatesare reliably recognized even in the case of very simply constructedcommunication units. In particular, no complex cryptographic protocolshave to be implemented in the communication units, because a cloning ofa communication unit is captured in the system via the chaining of datapairs.

In a particularly preferred embodiment of the method according to theinvention a respective first data set includes a time information itemwhich relates to the carrying out of the data transaction using thisfirst data set. Here, several parallel chains are detected in a simplemanner in that the data pairs are arranged according to the timesequence of the time information items of their first data sets, and thepresence of parallel chains is ascertained when a second data set of adata pair differs from the first data set of a data pair directlypreceding in the time sequence. The concept of time information is to beunderstood broadly here and does not have to include an explicit timespecification. E.g., a time information item can be coded by consecutivesequence numbers.

In a further, particularly preferred embodiment of the invention therespective second data sets are transferred without additionalinformation items being added by the given communication unit. Thus, themethod can be particularly easily implemented, because the respectivecommunication units must merely send previously received data sets anewwithout any further processing.

In a further configuration of the method according to the invention, thefirst data set is transferred within the framework of a data transactionin combination with a write command which specifies a first memoryaddress in a memory of the given communication unit, whereupon the firstdata set is stored at the first memory address.

Here, simple write commands (“write”), as they are known for examplefrom the standard ISO/IEC 18000-63 (the previous ISO/IEC 18000-6C), canbe used.

In a further embodiment of the invention, the above-described storing atthe first memory address is merely temporary. That is to say, before thefinal storing at a second memory address the first data set is stored atthe first memory address only temporarily, namely until the first dataset was subjected to a check.

In a preferred variant, upon this check the first data set is comparedwith the first data set stored upon the most recently carried out datatransaction and/or a signature is verified which was added to the firstdata set by the reading device. In the case of a sufficient differencebetween the first data set and the first data set stored upon the mostrecently carried out data transaction and/or upon a successfulverification of the signature, the first data set is finally stored atthe second memory address. Otherwise, the first data set is discarded,so that the data transaction was not successfully completed. Thisvariant avoids a multiple storing of the same or similar data sets. Thecriterion of sufficient difference can be suitably defined depending onthe embodiment. E.g., the criterion of sufficient difference can relateonly to certain fields in the data set. That is to say, a difference inthe other fields does not represent a sufficient difference leading tothe storage of the data set.

In a further configuration of the method according to the invention,within the framework of a data transaction the first data set of thepreceding data transaction, which has been stored at the second memoryaddress, is read out as a second data set by means of a read command andtransferred to the reading device. For this there can again be employeda simple conventional read command, as it is described for example inthe above-mentioned standard ISO/IEC 18000-63.

In a further configuration of the method according to the invention, forseveral consecutive data transactions carried out in the past therespective first data sets are stored in the given communication unit,preferably in a circular buffer or FIFO memory. This permits a largernumber of data transactions to be held in the communication unit and tobe read out as needed. Preferably, these first data sets can be read outto a predetermined memory address in the given communication unit byconsecutive read commands. In particular, the predetermined memoryaddress can employ a pointer, which points to the data set to becurrently read out, the pointer being incremented after the readout ofthis data set so that upon the next read command the next data set isread out.

In a particularly preferred embodiment, the invention is used for asystem which employs several reading devices, the data pairs resultingfrom the data transactions of the respective reading devices beingtransferred to the computer unit, which in this case is configured as acentral computer unit which is managed for example by the operator ofthe system. In this computer unit there is then effected the evaluationof the data pairs according to the invention on the basis of thedescribed chaining, so that duplicated communication units arerecognized hereby.

The method according to the invention is used in particular in thesystems for capturing services as already described above, the provisionof the service being captured via the data transactions. In aparticularly preferred embodiment, the system is a ticket system for thepublic transport, the first data sets respectively containing a pathinformation item, which includes in particular a route and a pathsection on the route and, where applicable, also further informationitems.

Besides the above-described method, the invention further relates to asystem comprising a number of reading devices and a plurality ofportable communication units. In analogy to the above method, duringoperation of the system, for a given communication unit there arecarried out a plurality of data transactions between at least onereading device and the given communication unit via a correspondingcommunication interface, wherein within the framework of a datatransaction a first data set is transmitted from a reading device to thegiven communication unit and is stored there and a second data set,which was stored in the given communication unit upon the most recentlycarried out data transaction, is transferred to the reading device.

The system contains a computer unit, by means of which a chaining ofdata pairs comprising the first and second data set of the respectivedata transactions is formed in such a way that for two consecutive datapairs the first data set of the one data pair corresponds to the seconddata set of the other data pair. Upon forming the chaining there is herechecked a criterion which is fulfilled when the chaining has severalparallel chains of data pairs or when two data pairs are found whichinclude different first data sets while the second data set is the same.A duplication of the given communication unit is recognized with thecomputer unit, when the criterion is fulfilled.

The above-described system according to the invention is preferablyconfigured such that one or several of the preferred variants of themethod according to the invention can be carried out with the system.

Exemplary embodiments of the invention will be described in detailhereinafter with reference to the attached Figures.

There are shown:

FIG. 1 a schematic representation of a first embodiment of the methodaccording to the invention;

FIG. 2 the structure of a first data set, which is employed in theembodiment of FIG. 1; and

FIG. 3 a schematic representation which depicts the memory management inthe RFID transponder which is used in the embodiment of FIG. 1.

Hereinafter there will be explained an embodiment of the methodaccording to the invention, which is based on a BiBo system of a publictransport (BiBo=Be in Be out). Users of this public transport carry acommunication unit in the form of a UHF-RFID tag or transponder. Such atag is based on the standard ISO/IEC 18000-63. This standard specifiescontactless transponders and reading devices which work in a UHF band ofabout 860 MHz to 950 MHz. The transponders are queried with a standardRFID reading device and then send back their identity by modulation ofthe high-frequency energy scattered by the antenna of the transponder(so-called backscatter method). The RFID tags draw the operating energyfrom the high-frequency energy radiated by the reading device, if theyare passive RFID tags. Moreover, the standard enables alsobattery-assisted passive tags (BAP=battery assisted passive).

Within the framework of the BiBo system via the contactlesscommunication between the RFID tag and the corresponding reading devicesin the means of transport, which a person having the tag employs, theroute of this person is captured. Upon a change of the means oftransport, the capturing of the route is continued with thecorresponding reading device of the new means of transport. In this way,data sets are generated which are transferred to the operator of theBiBo system, who can thus capture the routes covered in a billing periodand can charge the owner of the tag therewith. Here it may come to anabuse to the effect that a relevant tag is cloned or duplicated by athird party and is abusively used in the BiBo system. The arising travelcosts are then billed to the owner of the original tag. It has to betaken into account here that the hardware of an UHF tag can only beinsufficiently protected against a cloning. This is due to the fact thatthe energy which a UHF tag can draw from the high-frequency field of thereading device is, in general, so low that, in most cases, the tag iscomposed of only the simplest circuits and thus no complex cryptographicprotocols or operations can be realized.

With the embodiments of the method according to the invention describedhereinafter there is realized a suitable mechanism for an efficientrecognition of cloned tags during operation of the BiBo system. FIG. 1shows the communication of a reading device 1 with a corresponding RFIDtag 2 in such a BiBo system. The communication is effected via thecontactless interface IF, via which information items are exchangedbetween the antenna 101 of the reading device 1 and the antenna 201 ofthe tag 2. The system includes a plurality of reading devices 1 whichare provided in the corresponding means of transport of the publictransport. These devices communicate with the corresponding tags withintheir range. Thus, for each tag there can be captured an informationitem with regard to the covered route in the form of corresponding datasets which are transferred by the reading devices to a central computerunit or data base 3. There the system operator then performs a billingof the trips covered in a certain billing period to the owners of therespective tags.

Within the framework of the communication between reading device 1 andtag 2, represented in FIG. 1, it is assumed that there was first carriedout an identification of the tag to the reading device based on theabove-mentioned backscatter method. There can further be used acryptographic protocol upon the identification, where applicable, suchas e.g. the protocol which is described in the above-mentioned print ofA. Arbit et al. After the identification the reading device 1 sends viathe interface IF by means of a write command Write@A1 a first data setDS(n) to the tag 2. The first data set is stored in a correspondingmemory S of the tag 2. This memory includes a first storage region A anda second storage region B, which are not specified in more detail inFIG. 1 but will be explained below with reference to FIG. 3.

FIG. 2 shows the structure of the first data set DS(n) which istransferred from the reading device 1 to the tag 2. The data setincludes a sequence number SN assigned by the reading device. It furthercontains a time T1 in the form of a date or a time of the day, wherebythe accuracy of the time does not need to exceed the order of magnitudeof a minute. Further, a route information item is deposited in the fieldLI, which contains the characteristics of a certain trip and tripdirection, e.g. within one day. Further, a field SE is provided whichspecifies the path section between two stops, at which the means oftransport was entered. The data set of FIG. 2 is further provided with asignature SIG, which is optional and not represented in FIG. 1 in thetransfer of the data set. The signature here can be generated with a keywhich is specific for the reading device. Likewise, a global signaturekey valid in the whole system can be employed, where applicable.

After the transfer of the first data set DS(n) to the tag 2 as well asafter corresponding check steps, which will be described below withreference to FIG. 3, according to FIG. 1 there is transferred a seconddata set DS(n−1) by means of the read command Read@A2 to the readingdevice. The second data set DS(n−1) here is the data set which wastransferred from the reading device 1 to the tag 2 upon the most recentdata transaction. In the BiBo application a data transaction normallytakes place when the means of transport is changed. The first and seconddata sets DS(n) and DS(n−1) exchanged within the framework of a datatransaction are stored as the data pair DP and transferred to thecentral computer unit 3 of the system operator. In so doing, theaccordingly generated data pairs are collected from all the readingdevices in the system. In the computer unit 3 the individual data pairsare then chained. In FIG. 1 by way of example there is depicted such achaining of data pairs. For reasons of clarity, only some of the datapairs are designated with the reference sign DP. The chaining is hereeffected such that a data pair is linked with a temporally later datapair, when the first data set of the data pair matches the second dataset of the temporally later data pair. This chaining is indicated inFIG. 1 with corresponding double arrows. All the data pairs whichcontain the index n form a continuous chain. And n−6, n−5, . . . , ndesignate consecutive points in time for the corresponding datatransactions of the tag 2.

In the scenario of FIG. 1 it is further to be taken into account that acloned tag 2′ was used parallel to the tag 2. This tag has also carriedout corresponding data transactions based on data sets DS(c), DS(c−1)etc. within the framework of the communication with the reading device1. The corresponding data sets or the data pairs resulting therefrom aredepicted in dotted manner in FIG. 1. As one can recognize, these datapairs can also be chained to each other, so that two parallel chains areformed, one chain consisting of the data sets for the tag 2 and theother chain of the data sets for the cloned tag 2′. In the embodiment ofFIG. 1 the forming of two parallel chains running independently side byside is ascertained via a temporal sorting of the collected data pairsDP. The list of these data pairs DP of FIG. 1 is sorted such that datasets which were generated later stand higher up in the list. Within theframework of the analysis of the sorting there will be recognized thatfor certain data pairs the second data set differs from the first dataset of the temporally preceding data pair. These cases are marked byserrated symbols z. The recognition of such cases allows the conclusionthat temporally parallel to the tag 2 there is employed a cloned tag 2′.As a consequence, the system operator will then block the correspondingtag 2 and thus also its clone from being further used, in order toprevent further abuse.

Besides the recognition of clones on the basis of the temporal sorting,duplicated tags are also detected in the embodiment of FIG. 1 when itturns out that there exist several data pairs which include differentfirst data sets but the same second data set. This situation can onlyoccur, when two identical tags are used in the system. In this case,too, the system operator will arrange for the tag to be blockedaccordingly.

The sequence number SN (FIG. 2) assigned by the reading device 1 can beused, where applicable, for the recognition of such cloned tags, whichafter their generation are always employed together with the originaltag. Without a sequence number the original tag and the cloned tag wouldmostly contain the same data sets, if the time T1 is only roughlyresolved. Nevertheless, they can be addressed separately via the readingdevice, which is ensured by the singularization method described in thestandard ISO/IEC 18000-63. By the additional inclusion of the sequencenumber SN generated by the reading device the data sets are stilldistinguishable, however, so that clones can be recognized upon thelater processing in the computer unit 3.

In the system of FIG. 1 the reading device, which communicates with thetags in the means of transport, carries out an inventarization in eachpath section of a trip and in this way can also ascertain where a taghas left the means of transport (or where it was present the last time).The tag stores only the first inventarization according to the route orline of the means of transport (field LI of FIG. 2). Subsequentinventarizations with the same route or line, but a different pathsection of the same trip are not stored. Upon an inventarization requestof the reading device in the form of the above-described first data set,the tag responds with the most recently stored second data set, whichcorresponds to the preceding trip (different line or route and thelike).

As already mentioned, the tags' data collected by the reading devicesare reported to a central computer 3 which links said data, whereapplicable, with various trip changes and calculates the fare basedthereon and charges it to the customer. With the above-described method,by chaining the data sets there can be recognized inconsistencies andthus cloned tags can be ascertained. A tag for which a clone wasrecognized leads to the blocking thereof and to an entry of the tag intoa blocking list which is distributed to the reading devices in thesystem. The owner of the blocked tag is notified of the blocking.Thereupon, the tag will no longer be accepted as a ticket in the system.

Hereinafter there will be described with reference to FIG. 3 a memorymanagement of the data sets processed in the tag 2 of FIG. 1. FIG. 3here shows analogous to FIG. 1 a scenario in which the data set DS(n)having a corresponding signature SIG is deposited in the tag 2 by thereading device 1 via the contactless interface IF by means of the writecommand Write@A1. From FIG. 3 the structure of the two memory regions Aand B of the memory of the tag is apparent. The address A1 specified viathe write command lies in the storage region A. There, the data setDS(n) is temporarily stored. Said write command is a conventional WRITEor BLOCK WRITE command of the standard ISO/IEC 18000-63.

Within the framework of the check CH there is first ascertained, whetherthe relevant data set was to be stored in the first place, becausememory space would be unnecessarily wasted, when several data sets arestored for the same transport process. Accordingly, the data set iscompared with the most recently stored data set DS(n−1). This data setis located at the memory address A2 according to FIG. 3. When the datasets differ, in a next step the signature SIG of the data set DS(n) ischecked. If a key specified for the reading device 1 was used therefor,the tag 2 first checks the certificate of the signature, from which itthen infers the public verification key. If a global signature key validin the whole system is employed, the step of certificate verification isomitted, which has the disadvantage, however, that the system will bebroken, if the signature key is compromised. When signature keys areused which are specific for the respective reading devices, thecertificate is normally issued with a short runtime, so that alreadyafter a sufficiently short time a compromised reader cannot generatevalid data sets any longer. The tag can here conclude from the timestamp of its most recently stored entry that the runtime of thesignature key has expired.

If, finally, upon the check CH there was recognized the need for storingthe data set DS(n) as well as successfully verified the signaturethereof, the final storing of the data set at the memory address A2 iseffected, thereby the data set DS(n−1) being replaced by the data setDS(n). Prior to this, however, the data set DS(n−1) is transferred viathe command Read@A2 to the reading device 1. Further, the data set DS(n)is deposited in the circular buffer or FIFO memory B at the memoryaddress B1, with the consequence that the oldest data set DS(n−x)deposited at the memory address Bx is overwritten and deleted. Thenumber of older preceding data sets in the circular buffer B can bedefined system-specifically and depends for example on the tag's averagefrequency of use, the billing period and quantities connected therewith.

As described above, within the framework of a data transaction thepreceding data set DS(n−1) is transferred via the interface IF to thereading device 1 for the formation of data pairs. For this purpose, theread command Read@A2 is employed, with which the memory address A2 isread out. For this again a conventional READ command of the standardISO/IEC 18000-6C can be employed. The read command can optionally bealready employed by the reading device so as to hereby ascertain whetherit must send a further data set to the tag for storage. In this way, thesystem throughput can be optimized, where applicable.

In the embodiment of FIG. 3 there is further provided a further memoryaddress A3, which permits the whole circular buffer B to be read out viaa READ command to the address A3. With each READ command a further olderdata set DS(n−2), . . . , DS(n−x) is output via the address A3. This isachieved by the pointer P, which after each readout of a data set pointsto the preceding data set in the circular buffer B. As the movementprofile of the user can be tracked via the access to the address A3,this function is reserved only for certain, explicitly authorizedreading devices, which for this purpose must authenticate themselves tothe tag. The authentication can again be effected via a public keyprotocol. With the help of the data read out from the circular bufferthere can be ascertained later which trips the user has made in therecorded period and thus where applicable checked a billing.

To avoid the circular buffer B to be overwritten very fast throughmultiple writing (e.g. by a plurality of different reading devices in apublic means of transport) the check CH already mentioned above isprovided, according to which it is ascertained whether the current dataset differs from the preceding data set. The tag here can be adaptedsuch that a data set is written into the circular buffer or at theaddress A2 only when a minimum requirement on the difference between thecurrent and the preceding data set is recognized. For example, there canoccur the case that all the reading devices in a public train transferthe same path number, train number or stop number in the data set. A tagwould then write a data set, after a check, into the circular buffer Bor at the address A2 e.g. only when train number or stop number in thedata set differ from a previously received data set. Train number andstop number are only exemplary embodiments. In practice, further checkcriteria, such as e.g. the vehicle identity, the signature, and the likeare conceivable.

The hereinabove described embodiments of the method according to theinvention have a number of advantages. In particular, a simple andefficient recognition of duplicated portable communication units or tagsis achieved without complex cryptographic protocols having to beimplemented in the tag. Rather, by the analysis of data pairs ofconsecutive data sets it is recognized in the system, whether a tag wascloned. The method has in particular advantages when UHF tags areemployed, which are operated via the energy of the reading device fieldand thus have not available a sufficient power supply for cryptographicmechanisms for the protection of the tag.

1-14. (canceled)
 15. A method for monitoring a system comprising anumber of reading devices and a plurality of portable communicationunits, wherein during operation of the system, for a given communicationunit there are carried out a plurality of data transactions between atleast one reading device and the given communication unit via acorresponding communication interface, wherein within the framework of adata transaction a first data set is transmitted from a reading deviceto the given communication unit and stored there and a second data set,which was stored in the given communication unit upon the most recentlycarried out data transaction, is transferred to the reading device,wherein in a computer unit: a chaining of data pairs comprising thefirst and second data set of the respective data transactions is formedin such a way that for two consecutive data pairs the first data set ofthe one data pair corresponds to the second data set of the other datapair; upon forming the chaining there is checked a criterion which isfulfilled when the chaining has several parallel chains of data pairs orwhen two data pairs are found which include different first data setswhile the second data set is the same; a duplication of the givencommunication unit is recognized when the criterion is fulfilled. 16.The method according to claim 15, wherein the communication units aretransponders which communicate via a contactless communication interfacewith the number of reading devices, wherein the transponders are inparticular RFID transponders including UHF-RFID transponders.
 17. Themethod according to claim 15, wherein a respective first data setincludes a time information item which relates to the carrying out ofthe data transaction using this first data set, wherein the data pairsare arranged according to the time sequence of the time informationitems of their first data sets and several parallel chains are detectedwhen a second data set of a data pair differs from the first data set ofa data pair directly preceding in the time sequence.
 18. The methodaccording to claim 15, wherein the respective second data sets aretransferred without additional information items added by the givencommunication unit.
 19. The method according to claim 15, wherein thefirst data set is transferred within the framework of a data transactionin combination with a write command which specifies a first memoryaddress in a memory of the given communication unit, whereupon the firstdata set is stored at the first memory address.
 20. The method accordingto claim 19, wherein before a final storing at a second memory addressthe first data set is first temporarily stored at the first memoryaddress, until the first data set was subjected to a check.
 21. Themethod according to claim 20, wherein upon the check the first data setis compared with the first data set stored upon the most recentlycarried out data transaction and/or a signature is verified which wasadded to the first data set by the reading device, wherein in the caseof a sufficient difference between the first data set and the first dataset stored upon the most recently carried out data transaction and/orupon a successful verification of the signature, the first data set isstored at the second memory address, and otherwise the first data set isdiscarded.
 22. The method according to claim 20, wherein within theframework of a data transaction the first data set of the preceding datatransaction, which has been stored at the second memory address, is readout as a second data set by means of a read command and transferred tothe reading device.
 23. The method according to claim 15, wherein forseveral consecutive data transactions carried out in the past therespective first data sets are stored in the given communication unitincluding a circular buffer or FIFO memory.
 24. The method according toclaim 23, wherein the respective first data sets of the consecutive datatransactions carried out in the past can be read out to a predeterminedmemory address in the given communication unit by consecutive readcommands.
 25. The method according to claim 15, wherein the systemincludes several reading devices and the data pairs resulting from thedata transactions of the respective reading devices are transferred tothe computer unit which centrally evaluates the data pairs in order torecognize duplicated communication units.
 26. The method according toclaim 15, wherein the system is a transport ticket system for a publictransport and the first data sets respectively contain a pathinformation including a route and a path section on the route.
 27. Asystem comprising a number of reading devices and a plurality ofportable communication units, wherein during operation of the system,for a given communication unit there are carried out a plurality of datatransactions between at least one reading device and the givencommunication unit via a corresponding communication interface, whereinwithin the framework of a data transaction a first data set istransmitted from a reading device to the given communication unit andstored there and a second data set, which was stored in the givencommunication unit upon the most recently carried out data transaction,is transferred to the reading device, wherein the system includes in acomputer unit by means of which: a chaining of data pairs comprising thefirst and second data set of the respective data transactions is formedin such a way that for two consecutive data pairs the first data set ofthe one data pair corresponds to the second data set of the other datapair; upon forming the chaining there is checked a criterion which isfulfilled when the chaining has several parallel chains of data pairs orwhen two data pairs are found which include different first data setswhile the second data set is the same; a duplication of the givencommunication unit is recognized when the criterion is fulfilled. 28.The system according to claim 27, wherein the system is configured forcarrying out a method for monitoring a system comprising a number ofreading devices and a plurality of portable communication units, whereinduring operation of the system, for a given communication unit there arecarried out a plurality of data transactions between at least onereading device and the given communication unit via a correspondingcommunication interface, wherein within the framework of a datatransaction a first data set is transmitted from a reading device to thegiven communication unit and stored there and a second data set, whichwas stored in the given communication unit upon the most recentlycarried out data transaction, is transferred to the reading device,wherein in a computer unit: a chaining of data pairs comprising thefirst and second data set of the respective data transactions is formedin such a way that for two consecutive data pairs the first data set ofthe one data pair corresponds to the second data set of the other datapair; upon forming the chaining there is checked a criterion which isfulfilled when the chaining has several parallel chains of data pairs orwhen two data pairs are found which include different first data setswhile the second data set is the same; a duplication of the givencommunication unit is recognized when the criterion is fulfilled.